X-ray reflectometry (XRR) is a technique for measuring the thickness of thin films in semiconductor manufacturing and other applications. The present invention relates to such a measurement system and provides for making adjustments to components of the system to improve the operation of the system.
There has been significant interest in developing x-ray reflectance techniques for analyzing thin films, and particularly thin metal films. Thin metal films are not easily analyzed using conventional optical metrology techniques that rely on visible or UV wavelengths since metal films are opaque at those wavelengths. X-rays are of interest since they can penetrate metals.
The basic concepts behind measuring thin metal films on a substrate using an x-ray reflectance technique are described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997, and incorporated herein by reference. As described therein, a beam of x-rays is focused to strike the thin metal sample over a range of angles from near grazing incidence to a few degrees. A photodetector array detects the reflected x-rays over a range of angles of incidence. In this configuration, interference effects are created between the x-rays which reflect off the upper surface of the sample and at the interface between the thin film layer and the substrate. These interference effects vary as a function of angle of incidence. A plot of the change in intensity of the x-rays detected at the photodetector as a function of angle of incidence reveals periodic fringes, the spacing of which is a function of film thickness. Additional film properties, such as density and surface roughness, can be inferred from other characteristics of the reflectivity profile, such as the fringe amplitude or the location of the critical angle (onset of total external reflection).
As with many systems, there are many trade-offs involved in the design parameters of an XRR system. For example, as the thickness of the films being measured increases, the spacing (as a function of angle of incidence) between the fringes becomes smaller. In order to be able to analyze such closely spaced fringes, it is desirable to maximize the resolution of the system. In particular, the spread of angles of incidence detected by any one pixel in the detector array should be as small as possible.
One drawback associated with increasing the resolution of the system is that the flux or amount of energy received by each pixel is typically reduced. Reduced flux results in a less favorable signal to noise performance which in turn increases the time needed for successful measurements. While the trade-off may be required to measure thicker films, this increase in time would be an undesirable, and unnecessary, penalty when measuring thinner films.
Typically systems are designed to balance the need to make measurements of thicker films which require higher resolution, with the need to make measurements of thinner films quickly and efficiently. The goal is to balance these competing factors so that the resulting measurement instrument will have a good balance between resolution and signal to noise performance. However, such systems do not allow physical characteristics of the measurement instrument to be adjusted to optimize them for measuring different films with a range of different thickness.
It was recognized by the inventors herein that an improved system would allow the operator the freedom to adjust the resolution to best suit the measurement of a particular sample. For example, when measuring very thin films, the fringe spacing is quite large and high resolution is less important. In such a case, it would be helpful if the user could adjust the system to increase the flux thereby improving signal to noise performance and measurement speed. One design approach for increasing the flux is to move the detector array closer to the sample. Another approach is to tilt the X-Ray source such that apparent width of the source, as imaged on the sample, is increased.
When measuring thicker films, the spacing between fringes is reduced. In such a case, having a high-resolution system is critical in being able to obtain accurate measurements. Therefore, it would be helpful if the operator could maximize resolution even if it meant that measurement time would be increased, since without sufficient resolution, information about the layer could not be derived at all. One design approach for increasing resolution is to move the detector array farther away from the sample. Another approach is to tilt the X-Ray source such that apparent width of the source, as imaged on the sample, is reduced.
In order to achieve these goals, the inventors herein propose an XRR system that includes one or more mechanisms that would permit the operator to adjust the resolution of the system for a particular measurement. In one embodiment, the operator is able to adjust the distance of the photodetector array from the sample. As this distance is increased, the resolution will be increased.
In another embodiment, the user is able to control the effective width of the x-ray probe beam imaged on the sample. The smaller the effective width of the x-ray probe beam, the higher the resolution. The effective width is controlled by adjusting the angle of the x-ray emission material.
Such a system can be implemented in a simultaneous multiple angle of incidence XRR system of the type described in the above-cited U.S. Patent. Further details of an embodiment of an XRR system developed by the assignee herein can be found in PCT Publication WO 01/71325 A2 published Sep. 27, 2001, and incorporated herein by reference (referred to herein as the ""325 application).